Implantable medical devices (IMDs) are implanted in patients to monitor, among other things, electrical cardiac activity, and to deliver appropriate cardiac electrical therapy, as required. IMDs include pacemakers, cardioverters, defibrillators, implantable cardioverter defibrillators (ICD), and the like. The electrical therapy produced by an IMD may include pacing pulses, cardioverting pulses, and/or defibrillator pulses to reverse arrhythmias (e.g., tachycardias and bradycardias) or to stimulate the contraction of cardiac tissue (e.g., cardiac pacing) to return the heart to its normal sinus rhythm. IMDs can also be used to perform cardiac resynchronization therapy (CRT). IMDs can alternatively, or additionally, provide for neurostimulation.
When IMDs are exposed to external magnetic fields, such as those produced by magnetic resonance imaging (MRI) systems, the magnetic fields may interfere with operation of the IMDs. For example, an MRI system can cause electromagnetic field interference with leads, electrodes, and/or other sensing circuitry of or attached to an IMD. These forces may induce electric charges or potentials on the leads and electrodes, which can cause over- or under-sensing of cardiac signals. For example, the charges may cause the electrodes and leads to convey signals to an IMD that are not cardiac signals but are treated by the IMD as cardiac signals. This may cause the IMD to falsely detect tachycardias (which do not actually exist), potentially causing the IMD to delivery anti-tachycardia pacing (ATP) or cardioversion/defibrillation shock therapy (when not actually necessary and may be pro-arrhythmic). In another example, the charges induced by MRI systems may induce sufficient noise in cardiac signals such that cardiac signals that are representative of cardiac events go undetected by an IMD. This may cause the IMD to not detect a normal sinus rhythm (which actually exists), potentially causing the IMD to delivery inappropriate pacing (when actually unnecessary and may be pro-arrhythmic). This may also cause the IMD to not deliver pacing therapy (when actually necessary), since it falsely believes there are intrinsic cardiac events ongoing.
An MRI system generally produces and utilizes three types of electromagnetic fields, which include a strong static magnetic field, a time-varying gradient magnetic field, and a radio frequency (RF) magnetic field, which can collectively be referred to as the electro-magnetic field from an MRI system. The time-varying gradient field and the RF field may be referred to as different parts of the time-varying electro-magnetic field. In other words, the time-varying gradient field and the RF field can collectively be referred to as the time-varying electro-magnetic field. The static field produced by most MRI systems has a magnetic induction ranging from about 0.35 Tesla (T) to about 4 T, but can be potentially higher (e.g., 7 T and 9 T MRI systems are sometimes used in research). More specifically, MRI systems may generate external static magnetic fields having different strengths, such as 0.35 T, 0.5 T, 0.7 T, 1.0 T, 1.2 T, 1.5 T, 3 T, 4 T etc. The RF field includes RF pulses. The frequency of the RF field is correlated to the magnitude of the static magnetic field to provide the best scanning result, with the frequency of the RF field being approximately 42.58e6*static field strength. For example, where the static magnetic field strength is 1.5 T, the RF is at 42.58e6˜1.5˜64 MHz; and where the static magnetic field is 3 T, the RF is at 42.58e6*3˜128 MHz. The time-varying gradient magnetic field, which is used for spatial encoding, typically has a frequency in the KHz range, but for many MRI sequences can have relatively high power in the sub-KHz range.
In order to safely operate while exposed to magnetic fields produced by MRI systems, IMDs may need to switch modes to an “MRI safe mode”, which is sometimes more succinctly as an “MRI mode”. When in an MRI safe mode, an IMD may change the algorithms, software, and/or logical steps by which cardiac signals are monitored, and/or by which pacing and/or other cardiac therapy is delivered. For example, an IMD may change which algorithms are used to identify an arrhythmia. Alternatively, the IMD may cease measuring or sensing cardiac signals.
The normal operational mode can be the operational mode of the IMD prior to it being switched to the MRI safe mode. Thus, for cardiac rhythm management (“CRM”) type IMDs, such as Brady and/or Tachy devices, for example, the normal operational mode is the CRM device's initially programmed mode. The term “MRI safe mode”, as used herein, can refer to any operational mode of an IMD that is a safe operational mode in the presence of the electro-magnetic fields generated by MRI systems. For example, for a Brady device (as well as a Brady engine in a Tachy device) an MRI safe mode might be a fixed-rate and/or non-demand (or asynchronous) pacing mode for a patient that needs pacing, or can turn off pacing for a patient that is not pacer dependent, as opposed to a rate-responsive and/or demand pacing mode. In some embodiments, an MRI safe mode can be both a non-demand mode (i.e., VOO) and a non-rate-responsive mode. Thus, in accordance with one embodiment, switching a Brady device to an MRI safe mode might entail mode switching to a VOO, AOO or DOO pacing mode.
The MRI safe mode to which the IMD is switched will typically depend on the normal operational mode of the IMD. For example, an IMD whose normal operational modes is a Dxx mode (e.g., a DDDR, DDD, DDI, or DVI mode) can perform a mode switch to DOO when exposed to an electro-magnetic field generated by an MRI system (i.e., the MRI safe mode can be a DOO mode). In another embodiment, for an IMD whose normal operational mode is a Vxx mode (e.g., a VDDR, VDD, VDI, or VVI mode), the MRI safe mode can be a VOO mode. In still another embodiment, for an IMD having an Axx mode as its normal operational mode (e.g., an ADDR, ADD, ADI, or AVI mode), the MRI safe mode can be an AOO mode. These are just a few examples, which are not meant to be all encompassing.
The MRI safe mode for a Tachy device might comprise turning-off tachy detection and/or therapy, as well as switching to a fixed-rate, non-demand pacing mode. In these embodiments, turning the tachy detection off will ensure that noise which might be induced on the device leads by an MRI scan is not mistaken by the device for a tachycardia, which might result in an inappropriate anti-tachycardia pacing (ATP) or shock during an MRI. Also, for CRM devices, there may be other modes of operation that are considered safe in an MRI environment.
Once the IMD leaves or is otherwise not exposed to the strong magnetic field from an MRI system, the IMD should preferably switch back to its normal mode of operation, which is also referred to as the normal operational mode. In the normal operational mode, the IMD may resume monitoring cardiac signals as the IMD 110 did before the IMD was exposed to a strong magnetic field from an MRI system. Exemplary normal operational modes and MRI safe modes were discussed above.
An IMD's failure to switch from its normal operational mode into an MRI safe mode, when it should have, may cause the IMD to inhibit necessary pacing, or delivery unnecessarily high voltage therapy or anti-tachycardia pacing, which may induce an arrhythmia. Further, failure of an IMD to switch out of an MRI safe mode and back to its normal operational mode, when it should have, may cause pacing that leads to non-optimal therapy, loss of rate-response, pacemaker syndrome, and/or other problems, and may cause no delivery of high voltage therapy or anti-tachycardia pacing to treat an tachyarrhythmia.
Some IMDs require that a clinician send a telemetry command to the IMD, via a special external programmer, in order to put the IMD in an MRI safe mode, as well as to switch the IMD out of the MRI safe mode and back to its normal operational mode. However, the needs for this special external programmer and for clinician training on using the external programmer are time consuming, costly and cumbersome. Further, this protocol may not be properly followed, or interfere with other procedures, e.g., in emergency situations, when the technician operating the MRI system is not aware that the patient has an IMD, and/or when an appropriate external programmer is unavailable.
Some IMDs have an automatic MRI (Auto-MRI) detection capability that enables an IMD to automatically detect when the IMD is exposed to an MRI field of an MRI system, thereby enabling the IMD to switch itself into and out of an MRI safe mode. For example, such an IMD can use a magnetic field sensor to detect when a sensed magnetic field exceeds a specified threshold, and the IMD can switch itself into an MRI safe mode when the threshold is exceeded, and can switch itself out of the MRI safe mode when the threshold is no longer exceeded. Exemplary types of magnetic field sensors that can be included within an IMD and used to perform automatic MRI detection include a giant magnetoresistance (GMR) sensor, a reed switch, and a Hall effect sensor.
False positive detections of a magnetic field by a magnetic field sensor (e.g., a GMR sensor, a reed switch, or a Hall effect sensor) of an IMD may cause the IMD to inappropriately switch from its normal operational mode to its MRI safe mode. This may cause the IMD to not detect a tachycardia (which actually exists), potentially causing the IMD to not delivery appropriate anti-tachycardia pacing (ATP) or defibrillation shock therapy (when actually necessary), as well as non-optimal bradytherapy. Accordingly, inappropriately switching to the MRI safe mode may inhibit potentially life-saving defibrillation therapy.
Therefore, a need still exists for IMDs, and methods for use therewith, that can detect when an IMD is within an MRI system, preferably with increased specificity.